Can you add something like a
version.txt on sf.net?
Example:Then you should add a check on the client for this file. ( every hour/minutes, users will be able to change this check-time from the options )
Everytime it will see that there is a newest version, it will show it somewhere on the main screen of the GUI.
Example:newer version avvailable: 0.3.1
Please update it!
It's quiet simple, but it help to have more change that majority of clients will always be updated.
You will be able also to add a more advanced auto-update feature.
EDIT:
even better
[version]
0.3.1
[message]
newer version avvailable: &v
Please update it!
With this you will be able to add also a different message every new version